Add esmvaltool develop command group

[alistairsellar]

Description

Add esmvaltool develop command group, with 3 new commands (see issue for plan)

• Closes #3633

• Link to documentation: TBA

• • *

Before you get started

• [ ] ☝ Create an issue to discuss what you are going to do

Checklist

It is the responsibility of the author to make sure the pull request is ready to review. The icons indicate whether the item will be subject to the 🛠 Technical or 🧪 Scientific review.

• [ ] 🛠 This pull request has a descriptive title
• [ ] 🛠 Code is written according to the code quality guidelines
• [ ] 🛠 Documentation is available
• [ ] 🛠 Tests run successfully
• [ ] 🛠 The list of authors is up to date
• [ ] 🛠 Any changed dependencies have been added or removed correctly
• [ ] 🛠 All checks below this pull request were successful

New or updated recipe/diagnostic

• [ ] 🧪 Recipe runs successfully
• [ ] 🧪 Recipe is well documented
• [ ] 🧪 Figure(s) and data look as expected from literature
• [ ] 🛠 Provenance information has been added

New or updated data reformatting script

• [ ] 🛠 Documentation is available
• [ ] 🛠 The dataset has been added to the CMOR check recipe
• [ ] 🛠 The dataset has been added to the shared data pools of DKRZ and Jasmin by the @ESMValGroup/OBS-maintainers team
• [ ] 🧪 Numbers and units of the data look physically meaningful

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To help with the number of pull requests:

• 🙏 We kindly ask you to review two other open pull requests in this repository